In recent years, there has been remarkable progress in light emitting elements in which a nitride semiconductor (hereafter called a “nitride”) is used to form a light emitting layer therewithin, a current is injected from the outside, and electrons and holes are caused to recombine and cause light emission within the light emitting layer. Further, applications in illuminating apparatuses are attracting attention, in which a portion of the light emitted from the light emitting element is used to excite a phosphor, and the white light obtained by mixing the light occurring due to the phosphor and the light from the light emitting element is used as a light source. However, as yet a device which satisfies requirements for high efficiency has not been obtained. The reasons for this, particularly when focusing on the process of obtaining white light using a phosphor, are principally two factors which lower the efficiency.
The first factor lowering the efficiency is the fact that a portion of the energy is lost due to wavelength conversion (Stokes loss). More specifically, excitation light emitted from the light emitting element, and absorbed by the phosphor, is wavelength-converted into light having an energy lower than the energy of the light arising from the light emitting element, and is emitted again to the outside. At this time, a loss occurs equal to the difference in the energies of the excitation light from the light emitting element and the emitted light from the phosphor, and so the efficiency is lowered.
The second factor lowering the efficiency is the reduced efficiency due to nonradiative recombination in the phosphor (the reduced internal quantum efficiency of the phosphor). More specifically, crystal defects existing within the phosphor function as nonradiative recombination centers. And, a portion of the carriers generated within the phosphor by excitation light are captured by the crystal defects without contributing to emission, and so cause a decline in the light emission efficiency of the phosphor.
Hence when a phosphor is used to obtain white light by passing through the two stages described above, a prominent decline in efficiency results, and impedes improvement of the efficiency of the light emitting element. The above explanation cites Patent Document 1, previously proposed by the present applicant. In addition, when a phosphor is used, hydrolysis due to moisture (a hydration reaction) occurs for sulfide system, silicate system, and halo-silicate system phosphors, and in addition rapid degradation occurs due to ultraviolet ray and other excitation light, so that there are the problems that reliability is poor and lifetimes are short. Further, when a phosphor is used there are the problems that color rendering and hues are lacking. That is, when realizing white light with high color rendering, at present the emission of red phosphors is weak, and there is a tradeoff between color rendering and light emission efficiency. On the other hand, in ultraviolet light emitting semiconductors, at present high-efficiency phosphors have not been obtained by a method of excitation of RGB three-color phosphors.
Hence among the current technology, there exist only methods using RGB three-color chips to realize white LEDs with high color rendering and high reliability. However, there are such problems as difficulty in designing an optical system in which color variation does not occur, and, with respect to cost, applying this technology to equipment including general-level illumination.
Hence this applicant proposes, by using the columnar crystal structures without using a phosphor to address the above-described technical problem, a compound semiconductor light emitting element capable of emission of multicolor light such as white light, made possible using a single chip. Specifically, by causing crystal growth of nuclei on a substrate at a temperature lower than the normal growth temperature of the columnar crystal structures, variation can be imparted to the nuclei. Thereafter, by causing growth of the columnar crystal structures in the usual way, variation can be imparted to the film thickness and composition of the light emitting layer as well, and the columnar crystal structures can be caused to emit light at different wavelengths. Patent Document 2 and similar describe growth of the columnar crystal structures.
A method of Patent Document 1 is an excellent method to realize a solid-state light source capable of multi-color light emission by a single simple growth process on the same substrate and at low cost. However, variation in growth is used to enable multi-color light emission, so that problems with precision arise when combining to obtain a desired hue, as in illumination applications and similar.